There are currently a variety of metallic fasteners available for attaching bone plates to fracture and surgery repair sites; spinal, cranial, and maxillo-facial plates have all been fastened using metal screws. The disadvantage of metal devices is that they are permanent and may need to be removed through a second surgical procedure. If they remain in the body, tissue atrophy and subsequent loosening of the device may occur.
Synthetic absorbable biocompatible polymers are well known in the art. Such polymers are typically used to manufacture medical devices, which are implanted in body tissue and absorb over time. Synthetic absorbable biocompatible aliphatic polyesters include homopolymers, copolymers (random, block, segmented and graft) of monomers such as glycolic acid, glycolide, lactic acid, lactide(d, 1, meso and mixtures thereof), .epsilon.-ccaprolactone, trimethylene carbonate and p-dioxanone. Numerous U.S. Pat. Nos. describe these polymers including 5,431,679; 5,403,347; 5,314,989; 5,431,679; 5,403,347; and 5,502,159.
With the advent of absorbable polymers has come a new generation of fasteners (screws, pins, etc.) that are designed to be gradually absorbed by the body as their functional use declines. Devices made of an absorbable material have the advantage that they remain only for the period of time required for healing.
There are a number of orthopedic rivets disclosed in the prior art.
U.S. Pat. No. 5,167,665 describes a blind rivet with a head on its proximal end and a center mandrel which has a bead or head distal to the head of the rivet. The combination is inserted into a predrilled hole in the bone and is secured under the cortical layer of bone by pulling the center mandrel proximally, thereby collapsing the bead to form a second head that remains under the cortical layer.
U.S. Pat. Nos. 5,268,001 and 5,725,529 describe a flexible sleeve member and an elongated element where the elongated element has an outer diameter that is greater than the inner diameter of the sleeve. The sleeve is inserted into a predrilled hole in the bone. Then the element is inserted into the sleeve forcing it outwardly to secure it to the bone.
U.S. Pat. Nos. 5,326,205 and 5,501,695 describe a fastener adapted to expand into bone. The fastener has a tubular body with a longitudinal opening with a head proximally and members that extend along its body distally. A puller with a head at its distal end is inserted into the tubular body. The assembly is inserted into a predrilled hole in the bone. When the puller is moved proximally, it compresses the members on the tubular body, crimping them so that the members engage the bone. The puller is then broken off at the head of the tubular body, leaving the device secured in the bone.
U.S. Pat. No. 5,480,403 describes a suture anchor that includes a rivet that has radially flexible legs proximally and a setting pin with a sharp tip on its end distal to the flexible legs of the rivet and a suture tied to its proximal end. The rivet and pin are inserted into a predrilled hole. The hole is drilled deeper than the depth of the rivet/pin to accommodate the pin since the pin is initially inserted beyond the distal end of the rivet. The pin is then pulled proximally, expanding the proximal legs of rivet which then engage the bone.
In another embodiment of U.S. Pat. No. 5,480,403, the suture anchor has a rivet and pin where the rivet has legs that extend distally. The rivet and pin are inserted into a predrilled hole. When the pin is pulled proximally, the rivet legs, being tapered such that the inner diameter of the rivet is less as one moves proximally towards the head of the rivet, radially expand to secure the suture anchor to the bone surface as the pin is pulled proximally. U.S. Pat. Nos. 5,713,903 and 5,720,753 describe a fastener having expanded legs at its distal end and a pin with a flared distal end set in a central bore at a depth greater than the length of the fastener legs. When the pin is moved proximally, the flared end of the pin expands the legs of the fastener to secure it to the bone. The pin is then detached at a notch at the head of the fastener. This fastener is specifically for securing soft tissue to bone.
These patents relate to devices which are inserted into a hole and then deployed by driving a member upwards, or proximally, through or into a radially flexible portion of the device. This requires that the hole be deeper than the depth of the deployed device in order to leave enough room to fully insert the device before deployment. Such designs are undesirable for surgical applications where only a thin layer of bone is available for attachment or it is not possible to drill beyond the depth of the thin bone layer for fear of damaging underlying soft tissues, such as in cranial and maxillo-facial surgical procedures. In such cases, the required length of the fastener for adequate fastening strength is very close to the total thickness of the bone.
U.S. Pat. No. 4,590,928 describes an anchor consisting of an elongated cylindrical body with a tapered coaxial channel and radially flexible legs and a pin insertable into the end having the head, the proximal end. The body is placed into a hole formed in bone and the pin is inserted into the channel to expand the legs radially into the surrounding bone. The body and pin are of biocompatible material and the body contains carbon fibers embedded in and extending longitudinally along its interior. Since carbon fibers are not absorbed by or resorbed within the body, this fastener is only partially absorbable.
This patent, U.S. Pat. No. 4,590,928, describes a fastener having a solid pin. It is well known that the material properties, such as Young's modulus of elasticity and the material yield stress, of bone tissue change significantly with age and vary significantly among individuals of the same age and sex. Having a solid pin
therefore limits the fastening ability of the device since the pin cannot deform if the surrounding bone is so stiff that the legs cannot expand radially outwards.
In such a case where the surrounding bone is stiffer than that for which the device was designed and the pin is solid, the large force required to drive the rigid pin into the tapered hole will be such that either the legs will be deformed and rendered useless or the pin will be crushed. A radially compressible or flexible pin that could deform radially inwards to fit the internal passage under high insertion load conditions would give the fastener better performance over a wider range of bone material properties.
Therefore, what is needed in the art is a novel absorbable fastener for cranial, maxillo-facial, and other reconstructive surgical applications in which the fastener is inserted into a hole having the same depth as the fastener and is deployed by driving a radially compressible member downwards, or distally, into a flexible body of the fastener from above. The current invention discloses such a device.